The present invention relates to a method for manufacturing a thin film photovoltaic device, and, particularly, to a method for improving the performance of a thin film photovoltaic device. It should be noted that in the present specification the terms "crystalline" and "microcrystalline" are intended to also mean partially amorphous material.
In recent years, photovoltaic devices using a semiconductor thin film have been developed extensively. The object of the development of these photovoltaic devices is to obtain a high quality semiconductor thin film formed on an inexpensive substrate in a low temperature process which affords the compatibility of low manufacturing costs with a high performance. Such a photovoltaic device is expected to be used in various applications such as solar cells and photo-sensors.
As an example of the photovoltaic device, a device in which a transparent conductive film, a thin film photovoltaic unit, a back transparent conductive film and a back metal electrode are successively laminated on a substrate has been known. The back transparent conductive film arranged between the thin film photovoltaic unit and the back metal electrode is formed to remove thermal strain caused by a difference in the coefficient of thermal expansion between the semiconductor thin film and the metal electrode thereby increasing the adhesion strength of the metal electrode to the thin film photovoltaic unit. Further the back transparent conductive film is formed also to prevent metal atoms of the metal electrode from diffusing into the photovoltaic unit. It is generally required for this back transparent conductive film to have a thickness of 10 nm to 1 .mu.M with high transmittance and a low resistance as low as 1.5.times.10.sup.-3 .OMEGA.cm or less.
Conventionally, the back transparent conductive film which meets such requirements has been formed, for example, by sputtering under conditions of a pressure of 2.times.10.sup.-2 Torr or less, a substrate temperature of 100 to 450.degree. C. and a discharge power of 500 to 1500 mW/cm.sup.2.
However, it has been found that the resulting photovoltaic device is not sufficiently improved in the photovoltaic characteristics when the back transparent conductive film is formed under the sputtering conditions as aforementioned.